1. Field of the Invention
The present invention relates to a method for producing a membrane-electrode structure used for a polymer electrolyte fuel cell.
2. Description of the Related Art
The petroleum source has been exhausted, and at the same time, environmental problems such as global warming due to the consumption of fossil fuel have increasingly become serious. Thus, a fuel cell receives attention as a clean power source for electric motors that is not accompanied with the generation of carbon dioxide. The above fuel cell has been widely developed, and some fuel cells have become commercially practical. When the above fuel cell is mounted in vehicles and the like, a polymer electrolyte fuel cell comprising a polymer electrolyte membrane is preferably used because it easily provides a high voltage and a large electric current.
As shown in FIG. 15, a membrane-electrode structure 10 has been known as a membrane-electrode structure used for the above polymer electrolyte fuel cell. The membrane-electrode structure 10 comprises: a pair of electrode catalyst layers 3, 3 formed by integrating by an ion conducting polymer binder, catalyst particles consisting of catalysts such as platinum supported by carbon particles such as carbon black; a polymer electrolyte membrane 1 capable of conducting ions, which is sandwiched between the electrode catalyst layers 3, 3; and diffusion electrodes 5, 5 that are laminated on the electrode catalyst layers 3, 3, respectively.
In the membrane-electrode structure 10, the electrode catalyst layer 3 is hydrophilic for the transference of protons or the elimination of water generated, and the like. On the other hand, the diffusion electrode 5 is configured such that a porous water-repellent layer 7 is formed on a carbon substrate layer 6 for the diffusion of reducing gas or oxidizing gas. The diffusion electrode 5 is laminated on the electrode catalyst layer 3 through the water-repellent layer 7. In the membrane-electrode structure 10, a separator acting also as a gas passage is further laminated on each of the diffusion electrodes 5, 5, so as to constitute a polymer electrolyte fuel cell.
In the polymer electrolyte fuel cell, one electrode catalyst layer 3 is used as a fuel electrode, and reducing gas such as hydrogen or methanol is introduced therein through the diffusion electrode 5 on the fuel electrode side. The other electrode catalyst layer 3 is used as an oxygen electrode, and oxidizing gas such as air or oxygen is introduced therein through the diffusion electrode 5 on the oxygen electrode side. Thus, protons are generated from the above reducing gas by the action of catalysts contained in the above electrode catalyst layer 3 on the fuel electrode side. The protons are transferred to the electrode catalyst layer 3 on the above oxygen electrode side through the polymer electrolyte membrane 1. Then, in the electrode catalyst layer 3 on the above oxygen electrode side, the protons are reacted with the above oxidizing gas introduced therein by the action of catalysts contained in the above electrode catalyst layer 3, so as to generate water. Thus, electric current can be generated by connecting the above fuel electrode with the above oxygen electrode through a conductor.
The above electrode structure has previously been produced by laminating the above diffusion electrode 5 on a laminated body that is obtained by attaching the above electrode catalyst layers 3, 3 to each side of the polymer electrolyte membrane 1, and then pressing them under heating (refer to e.g., Japanese Patent Laid-Open No. 2001-345110).
However, the above conventional production method has a problem in that since the diffusion electrode 5 is laminated on the electrode catalyst layer 3 that is hydrophilic through the water-repellent layer 7, even though they are pressed under heating, a sufficient adhesiveness might not be obtained between the electrode catalyst layer 3 and the diffusion electrode 5. If a sufficient adhesiveness is not obtained between the electrode catalyst layer 3 and the diffusion electrode 5, when a polymer electrolyte fuel cell comprising the above membrane-electrode structure 10 is produced, its resistance overvoltage increases, resulting in a decreased power generation efficiency.